Servo systems for positioning a load at a commanded position are known. A conventional servo system for positioning a load controls the actuation of a prime mover such as an electrical motor to cause a load which may be variable to move to a commanded position. A position sensor associated with the motor shaft or the load generates a position signal which is compared to a commanded position. An error signal which is equal to the difference between the commanded position and the actual system is applied to a proportional amplifier which controls the activation of the prime mover. Although the system contains an integration element in the prime mover, non-linearities, such as static and Coulomb friction, backlash in gear trains and quantization effects cause a positioning system of the foregoing type to have a poor steady state accuracy.
Positioning error in a system of the foregoing type may be decreased by increasing the gain of the control loop driving the prime mover. However, increased gain has the disadvantage of rendering the system oscillatory. At steady state, a finite position error will be present which can be exacerbated by an external disturbance acting on the load or a non-linearity in the load and/or prime mover.
FIG. 1 illustrates a prior art position control system 10 for positioning a load 12 at a commanded position. The load 12 is acted upon by an external force, such as a wind current, acting upon a flight control surface or the load or prime mover 16 contains non-linearities. The position of the load 12 is sensed by a position sensor 14 which may be associated with the load or an output shaft 18 of a prime mover 16. The position detector 14 produces a position signal which is applied to a summer 20 which computes the difference between the commanded position and the actual sensed position. The resultant output error signal is applied to a proportional amplifier 22.
The performance characteristic of the system 10 of FIG. 1 is acceptable when the external force and/or non-linearities are not present in the load 12 or prime mover. However, the presence of these effects prevents the system from achieving a high level of performance necessary for applications such as actuators for flight control surfaces of high performance aircraft.